Containers of various sizes bear identifying marks describing their structure or contents. Cardboard boxes are labeled with test data such as edge-crush strength. Retail packaging carries advertising and marketing messages for potential buyers. Intermodal containers (shipping containers) carry ISO 6346 serial numbers and size codes. Indicia and markings on containers are used to determine how to handle a container and to dispatch it to a desired location. These indicia are generally printed visually. However, visual indicia can wear off over time or be covered over with graffiti. There is therefore a need for a way of marking containers that retains the ease-of-use of visual indicia but does not exhibit some of the same limitations.
RFID tags are sometimes used to identify containers of products, e.g., products used in various commercial or industrial processes. RFID tags respond to an external RF signal transmitted wirelessly from an RFID reader with a stored identification code or other data. The term “reader” is customary and does not imply that the reader only listens; indeed, RFID readers generally communicate bidirectionally. RFID readers and tags can communicate using, e.g., the EPC Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz, Version 1.2.0, Oct. 23, 2008, incorporated herein by reference. A container with a tag affixed thereto is referred to herein as a “tagged container.” Tags on containers can carry information about the type of products in those containers and the source of those products.
However, RFID tags generally respond to any RFID reader within range. Containers, especially shipping containers, are often densely packed in a given location. For example, container ship MV Emma Mcersk can hold approximately 11,000 twenty-foot long intermodal containers (11,000 TEU). There is, therefore, a need for a way of selecting which RFID tag a reader should communicate with. There is also a need for a way of confirming that the RFID tag being communicated with corresponds to a particular container.
Various schemes use directional antennas for these purposes. However, directional antennas are not effective with configurations in which multiple RF-transparent tagged containers are arranged along a line extending from the reader, e.g., cardboard boxes loaded on a pallet. In these configurations, it is difficult to determine which container along the line is being read. Moreover, without a way of confirming that a tag being read corresponds to a particular container, an intruder can respond to a reader pretending to be the tag for a particular container. This is referred to as “spoofing.” If measures are not taken against spoofing, a spoofer can falsify identification data for a container, possibly exposing shipping companies to liability for mis-shipped goods. This problem is particularly noticeable in relatively less-controlled environments such as freight yards, as compared with factories or other tightly-controlled environments. A spoofer can stand outside the fence of a shipyard and, with a sufficiently powerful RFID reader, spoof RFID tags at a considerable distance.
There is, therefore, a continuing need for a way of reliably determining information about a container, and specifically of reliably communicating with a particular selected tagged container.